Because the Earth rotates on its axis, objects in the sky outside of the Earth's atmosphere, such as the moon, planets and stars, seem to rise in the east and move across the sky, and then set in the west.

German Equatorial Mounting Polar Axis

The Sun, Moon, planets, and all objects in the solar system, as well as nebulae, clusters, and stars in our own galaxy, and indeed even all of the other galaxies in the sky, follow this path across the sky as the Earth rotates beneath.

"Equatorial" telescope mountings have two axes, a polar axis and a declination axis, to help compensate for the Earth's rotation and aim at objects in different parts of the sky.

The polar axis of an equatorial mount should point at the north celestial pole and the process of making this so is called "polar aligning". This means making the polar axis of the mount parallel to the Earth's axis of rotation. This greatly simplifies the tracking of a celestial object across the sky with the motion of only one axis of the telescope's mount in right ascension.

For visual work, polar alignment is not that critical. An equatorial mount can simply be set up and the polar axis roughly aligned on Polaris, the North Star. Dobsonian telescopes do not have to be polar aligned at all.

The newer computer controlled altazimuth mounted telescopes can track an object through built-in sophisticated mathematical calculations that are transparent to the user and move the telescope in both the altitude and azimuth. However a computer driven altazimuth mounted telescope cannot take long-exposure photographs without the use of an expensive and relatively rare field de-rotator.

For more accurate alignment, the aim of the polar axis of the mount has to be adjusted in two planes, the azimuth and the altitude. The azimuth corresponds to the directions on a compass, and the altitude is simply the elevation above the horizon, in degrees.

The mount can easily be aligned in altitude to match the latitude of the observing site, and the azimuth adjusted with a compass to find north. The offset for the deviation between magnetic north and true north can be corrected with information from published sources or a pilot's sectional map for the observing location.

For long-exposure deep-sky astrophotography, polar alignment is much more critical, especially at remote locations with a portable setup. If the mount is not polar aligned correctly, field rotation will result. Stars at the edges of the frame of the image will appear to rotate around the guide star.

The Drift Method

Use of a polar alignment scope built into the mount will help speed up alignment, but "drift" aligning will probably also be necessary for critical work.

A star that is monitored at high power (200x) in a guiding eyepiece with cross hairs most probably will not stay in one exact location in the field. There are several different reasons for this.

The star will usually seem to bump around a bit if the seeing is not that good. It can move all over the place on a very short time frame if the seeing is really bad.

Even if the seeing is excellent, the star will slowly drift from its original location. It can drift east - west due to inaccuracies in the right ascension gear and drive train that move the telescope to compensate for the Earth's rotation.

This is normally associated with periodic error, so called because the error in drift will coincide with the period of rotation of the worm gear. If a star is carefully monitored, the star will move one way for about 1/2 of the period of the worm, and then move back the other direction until it has returned to its starting position. This movement will usually be gentle and slow, but there can be quick jerks and movements from erratic error depending on the quality of the worm, gear, and components. For excellent mounts, this periodic error can be as little as a few arc seconds. For mediocre mounts, it can be as large as several minutes of arc.

For long-exposure deep-sky astrophotography, this periodic error must be guided out by either manually by the photographer with a high-power cross hair eyepiece or automatically with a CCD auto-guider such as the SBIG ST-4 or STV.

If the mount is not polar aligned to good accuracy, there will also be a slow north or south drift in declination.

Drift polar aligning is accomplished by monitoring the declination drift of a star at high power in the eyepiece and adjusting the polar axis of the mount based on the direction of drift.

Two corrections are necessary based on two observations: one of a star on the meridian for the azimuth of the polar axis of the mount, and one of a star near the eastern or western horizon for the elevation of the polar axis.

While monitoring the drift, any east - west movement is ignored or guided out by corrections in right ascension only. It is important that no corrections be made in any north - south declination drift because this drift will indicate which we have to move the mount to achieve more accurate polar alignment.

Level the Mount

The azimuth plane of the mount does not have to be perfectly level to successfully drift align, however, it is certainly easier if the mount if made as level as possible when it is set up. If the mount is not level, any adjustments in azimuth or altitude will cause an error in the alignment of the other component.

To prove this, let's try a thought experiment. Imagine a mount where the azimuth plane is not parallel to the ground (i.e. level). Take an extreme example to make it easy to visualize, say imagine the azimuth is perpendicular to the ground. Then any azimuth adjustment will obviously affect the altitude also. If the mount is not level, the azimuth must be somewhere between 90 degrees perpendicular to the ground and parallel to the ground. Any azimuth adjustments in this case will cause errors in altitude, just to a lesser extent than in our extreme example.

If the mount is not level (and it almost never is perfectly level), repeated iterations of a star on the meridian and horizon should be performed until there is no drift at all for 5 or 10 minutes. However, the closer to level that it is, the less iterations will be necessary.

For the drift method of polar alignment, it is not necessary for the declination and polar axis to be exactly at right angles. Nor does it matter where the optical axis of the scope is. The only thing that matters in correct polar alignment is that the polar axis of the mount ends up parallel to the axis of the Earth's rotation.

Determining Directions in the Eyepiece

The directions east and west correspond to the right ascension of the mount. If you stand facing north, the stars ascend (or rise), to your right, in the east, due to the Earth's rotation about its axis,. Right ascension, get it? If you move the scope in right ascension, the star will move east or west in the eyepiece.

The directions north and south correspond to the declination of the mount. If you move the mount in declination, a star in the eyepiece will move north or south.

Here is the absolutely simplest way to determine directions in the eyepiece, the orientation of the eyepiece and diagonal (if one is used) do not matter:

East - West: Turn off the right ascension drive, the stars drift to the west.

North - South: Nudge the tube in declination towards the north, and the stars move towards the south in the eyepiece.

The directions up and down depend on the orientation of the eyepiece, your head, and the diagonal if any, and can become quite confusing.

Step 1. The Meridian Correction for the Azimuth of the Polar Axis

In the drift alignment method, a star, roughly at the intersection of the meridian and the celestial equator, is watched at high power in a cross-hair eyepiece. The declination drift here will indicate the mount's accuracy in its azimuth alignment.

The meridian is an imaginary line that runs from the north point on the horizon, through the zenith overhead, to the south point of the horizon. The line of the meridian is a line of right ascension. All lines of right ascension run north - south, but only the meridian also runs through the zenith. Lines of right ascension are the equivalent of lines of longitude on a globe of the Earth.

The celestial equator is the imaginary line that runs from east to west at a 90 degree angle to the celestial poles. For example, at 40 degrees north latitude, the celestial pole is 40 degrees above the northern horizon. The celestial equator is 90 degrees from that: on the meridian it is 130 degrees above the northern horizon and 50 degrees above the southern horizon.

Declination corresponds to lines of latitude on a globe of the Earth. Latitude is the angular distance north or south from the equator.

The star does not have to be exactly on the celestial equator, it can be a number of degrees, 10 or 20 or so, north of the celestial equator for this correction. Technically, drift due to polar axis misalignment in azimuth is independent of the declination of the star, but a star between the zenith and celestial equator is easiest to use for polar alignment purposes.

The cross hairs of the eyepiece are first aligned east-west and north-south. Slew the star at high speed with the telescope controls and rotate the eyepiece until the star's movement parallels one of the cross hairs. Then note the cardinal directions with the method described above.

A high power will indicate any drift in a shorter amount of time than a lower power eyepiece, so the higher the magnification, the better.

The star is placed on the cross hair that is aligned east-west with the right ascension of the telescope. The star's drift due to polar mis-alignment off this cross hair will either be north or south, and this drift is monitored.

The star should be watched for 5 - 10 minutes, but in the beginning of the process when the alignment is at its worst, it's drift will be readily apparent in a short amount of time.

This drift in declination, either north or south in the eyepiece, will indicate which way the azimuth of the polar axis needs to be adjusted.

On the meridian correction, if the star drifts South off the cross hair, the azimuth of the polar axis is too far east. Move the azimuth of the polar axis of the mount in a counterclockwise direction (to the west).

If the star drifts North, the azimuth of the polar axis of the mount is too far to the west, and needs to be rotated clockwise to the east.

Recenter and monitor the drift and repeat until there is no drift at all for 5 minutes.

If the mount is grossly out of polar alignment at the beginning of this process, it may take quite a while to zero in with enough accuracy until the star does not drift at all off the line for 5 minutes. So, be patient, and while learning drift alignment in the beginning be prepared to spend a considerable amount of time practicing and perfecting this technique.

Even with the experience I have with this method, it usually takes me about an hour to get really good polar alignment with the drift method, even after using a polar alignment scope built for the mount that get's me pretty close to start.

Note that drift in right ascension can be completely ignored. Any drift in right ascension is strictly a function of the quality of the drive of the mount and its periodic error, and not the polar alignment.

Step 2. The Horizon Correction for the Altitude of the Polar Axis

Now find a star about 15 - 20 degrees above the eastern horizon, somewhere between the celestial equator and 20 degrees north of it. If you have a clear horizon, you don't even have to change the declination of the telescope from what you used for the azimuth correction - simply slew it in right ascension and find a star somewhere above the eastern horizon.

Stars too close to the horizon will be adversely affected by differential atmospheric refraction and poor seeing.

A star on the western horizon can be used, but the directions given below must then be reversed.

Again, align the cross hairs east-west and north-south, and place the star on the cross hair that runs east-west, and watch for its drift off the line to the north or south.

On the horizon correction for the altitude of the polar axis, if the star drifts south, the polar axis is too low and needs to be elevated.

If the star drifts north, the polar axis is too high and needs to be lowered.

Recenter and monitor the drift and repeat until there is no drift at all for 5 minutes.

After completing the meridian and horizon checks, go back to the meridian and check it again.

(Note: if the eastern and western horizons are blocked it is possible to do the altitude adjustment by drifting on a star within about 10-20 degrees of Polaris, due East or West.)

A Shortcut to Remembering

I combine these corrections into one easy-to-remember saying...

"S - E", and "S - L".

Which means, "South - East", and "South - Low".

If the meridian star drifts SOUTH (the polar axis is too far) EAST

And, if the eastern horizon star drifts SOUTH (the polar axis is too) LOW.

We can shorten this to just "S - E - L" since the drift is south in both cases to just .

This combines both the azimuth and altitude corrections for the mount. In both cases, all you have to remember is what to do if the star drifts SOUTH.

Obviously, if the star drifts north, the correction is just the opposite.

Want to learn more about DSLR astrophotography?

If you like the information you have read here, I have three books that you may find of interest. The first is aimed at beginners in astrophotography. The second is aimed at more advanced deep-sky imagers. The third is for astrophotographers who want to learn how to use their DSLR cameras for high-resolution planetary imaging.

If you think there is a lot of information here on these web pages, just wait until you see how much more there is in these books!

This book on CD-ROM for beginning astrophotographers explains how to take beautiful images with your digital single lens reflex (DSLR) camera using simple step-by-step techniques that anyone can learn.

You will see how easy it is to take great pictures with very modest equipment and basic methods that are within everyone's ability.

With this book you will learn how to take amazing images of the night sky with your DSLR camera.

This book on CD-ROM will show you how to take planetary images with your Live-View equipped DSLR. It explains the basics of high-resolution planetary imaging and gives step-by-step directions on how to shoot exciting pictures the Sun and Moon and fascinating planets like Jupiter, Saturn and Mars.

It also tells you on how to process your images in programs like RegiStax and AutoStakkert!, with step-by-step directions that will produce beautiful results.

The CD-ROM also includes more than 100 minutes of video tutorials on image processing.

This book on CD-ROM is will help you answer the question "what should I shoot tonight?"

It will provide you with detailed information and examples of the many beautiful objects in the deep sky that you can photograph with your own equipment.

A master list of objects includes 500 of the best and most photogenic galaxies, nebulae, supernovae remnants, stars, star clusters and constellations. This list can be sorted by object name, object type, catalog number, constellation, right ascension and focal length.

Images of more than 275 select objects visible from the northern hemisphere are displayed on individual pages with photographic information and details about these objects.

All-sky constellation charts are clickable with links to individual constellation images. These, in turn, have objects plotted on them that link to object pages.

A local sidereal time calculator will tell you when objects are on the meridian where they are highest in the sky and best placed for photography.

These books will help you to avoid those bad practices that lead to poor images. I made just about every mistake you could make when I was first starting out and did not know what I was doing. You don't have to make these same mistakes. You too can learn the secrets of deep-sky astrophotography!

Don't waste your long and hard efforts at astrophotography - find out how thousands of others just like you have gotten excellent results by using these books.